Integrated optical devices for directly processing optical signals have become of greater importance as optical fiber communications increasingly replace metal cable and microwave transmission links. Integrated optical devices can advantageously be implemented as silica optical circuits having compact dimensions at relatively low cost. Silica optical circuits employ integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging", 7 J Lightwave Technol., pp. 1530-1539 (1989) (Henry et al. reference), which is herein incorporated by reference.
Typically, in silica optical circuits, a silicon substrate is provided with a base layer of SiO.sub.2, and a thin core layer of doped silica glass is deposited on the SiO.sub.2 layer. The core layer can be configured to a desired waveguide structure using standard photolithographic techniques. Then, a layer of doped silica glass is deposited over the core layer to act as an upper cladding. Further, numerous passive optical circuit components have been formed within conventional silica optical circuits to desirably provide signal processing in addition to optical signal routing within the silica optical circuit structure. Examples including, for example, low-pass, high-pass, band-pass and notch filters, couplers, multiplexers and demultiplexers. Configurations of typical passive optical components formed within silica optical circuits are described in, for example, H. M. Presby, "Silica Integrated Optical Circuits" (SPIE Optical Engineering Press, Bellingham, Wash. 1996).
However, typical optical circuit applications require passive as well as active devices, such as optical signal detectors and transmitters as well as modulators. In order to provide such applications, conventional optical circuits often interconnect active devices with passive optical devices formed within silica optical circuits using optical fibers. In such configurations, waveguides extend to edge surfaces of the silica optical circuits where edge connectors attach the optical fibers. However, the attachment of the edge connectors undesirably increases circuit fabrication costs and the resulting configuration is typically undesirably larger than the silica optical circuit requiring greater space in an associated optical system.
U.S. Pat. Nos. 5,135,605 and 4,750,799 describe fabrication techniques for producing hybrid integrated optical circuits in which active optical components are mounted on a top surface of a silica optical circuit. These hybrid integrated optical circuits have relatively compact dimensions compared to the previously silica optical circuits interconnected with active devices by optical fibers. In the hybrid integrated optical circuits, turning mirrors are positioned under a mounted active device and proximate an end of a planar waveguide to enable an exchange of optical signals between the active device and the planar waveguide. Conventional turning mirrors have reflective surfaces positioned opposite an end surface of the planar waveguide and at a 45.degree. angle relative to the direction of the waveguide as well as a top surface of the circuit.
A cross-sectional side view of an exemplary optical circuit 1 having a conventional turning mirror configuration 5 is illustrated in FIG. 1. In FIG. 1, a planar waveguide 10 is formed between respective cladding layers 15 and 20. The cladding layer 20 is disposed on a substrate 25, such as a silicon substrate. The turning mirror 5 is also positioned on the substrate 25 and has a reflective surface 7 at a 45.degree. angle relative to the waveguide 10. A device 30, such as an optical signal detector and/or transmitter, is positioned on the mirror 5 and cladding layer 15. The device 30 is positioned to transmit an optical signal that deflects off the mirror surface 7 and into the waveguide 10 or receive an optical signal propagating through the waveguide 10 that is reflected by the mirror surface 7. An exemplary path for a light signal to travel between the device 30 and planar waveguide 10 is depicted by dashed line 35.
In accordance with the hybrid integrated optical device fabrication technique of U.S. Pat. No. 4,750,799, pre-fabricated mirror and waveguide components are secured to the substrate. However, such a fabrication technique is prohibitively expensive in a mass fabrication environment. In contrast, the fabrication technique of U.S. Pat. No. 5,135,605 more advantageously forms cladding and planar waveguide layers on a substrate. Then, a multi-step etching process is employed to create the profile of the turning mirror reflecting surface and waveguide end surface at the desired positions in the circuit. Such an etching technique reduces circuit fabrication costs as well as enables the formation of a greater number of turning mirrors in area of the integrated optical circuit.
Nevertheless, a need exists for less complex turning mirror fabrication techniques that can be implemented at relatively low cost.